Process for stabilization of hydrogen silsesquioxane resin solutions

Abstract: A stabilized solution of hydrogen silsesquioxane resin is disclosed comprising 100 parts by weight solvent, 0.1 to 100 parts by weight hydrogen silsesquioxane resin and 0.002 to 4 parts by weight acid. These solutions are useful for forming coatings on substrates.

Description: The present invention relates to solutions of hydrogen silsesquioxane resin which are stable on extended storage. These stable solutions are obtained by incorporating small amounts of acids in the solution. Hydrogen silsesquioxane resin (H-resin) is known in the art to be a useful precursor to silica-containing ceramic coatings. For instance, U.S. Patent No. 4,756,977 describes a process for forming such a coating in which H-resin is diluted in a solvent, applied to a substrate and ceramified by heating. Various methods for producing H-resin are also known in the art. For example, U.S. Patent No. 3,615,272 describes a process which comprises adding trichlorosilane to a reaction medium comprising sulfuric acid and an aromatic hydrocarbon, washing the reaction mixture until neutral and recovering the condensed H-resin. Despite the fact that the reference teaches hydrolysis and condensation in an acid, the resultant product is neutralized to prevent gelation. U.S. Patent No. 5,010,159 teaches a similar process in which a hydridosilane is reacted in an arylsulfonic acid hydrolysis medium followed by separating the resultant H-resin and then contacting it with a neutralizing agent. The shelf life of the above solvent-diluted H-resins can be unreliable if the resin undergoes crosslinking and increases in molecular weight. We have now found that the incorporation of an acid in the H-resin solution limits the possibility of changes in the molecular weight of the resin. The present invention provides a method of stabilising a hydrogen silsesquioxane resin solution comprising 0.1 to 100 parts by weight hydrogen silsesquioxane resin in 100 parts by weight solvent characterised in that 0.002 to 4 parts by weight acid are added to said resin solution. At the time of our invention, the predominant thought within the prior art was that solutions of H-resin must be completely neutralized to prevent any degree of gelation during storage. This belief was a substantial prejudice against the applicant's present claims which are based upon our unexpected finding that such solutions can, in fact, be stabilized for storage by the addition of small amounts of acids. As used in this disclosure, the expressions "stable" or "stabilized" H-resin solutions are used to describe those solutions in which the molecular weight of the H-resin changes less over time than those which have not been stabilized. It is preferred that the "stable" solutions not gel. It is more preferred that the weight average molecular weight (Mw) of the "stable" solutions not change more than 100%. It is most preferred that the Mw of the "stable" solutions not change more than 25%. The H-resins which may be used in this invention include hydridosiloxane resins of the formula HSi(OH) x (OR) y O z/2 , in which each R is independently an organic group or a substituted organic group which, when bonded to silicon through the oxygen atom, forms a hydrolyzable substituent, x 0-2, y = 0-2, z = 1-3, x + y + z = 3. Examples of R include alkyls such as methyl, ethyl, propyl and butyl; aryls such as phenyl and alkenyls such as allyl or vinyl. As such, these resins may be fully condensed (HSiO 3/2 ) n or they may be only partially hydrolyzed (i.e., containing some Si-OR) and/or partially condensed (i.e., containing some Si-OH). Although not represented by this structure, these resins may also contain a small number (eg., less than about 10%) of silicon atoms which have either 0 or 2 hydrogen atoms attached thereto due to various factors involved in their formation or handling. The above H-resins and methods for their production are known from the aforementioned patents. For example, U.S. Patent No. 3,615,272 teaches the production of a nearly fully condensed H-resin (which may contain up to 100-300 ppm silanol) by a process comprising hydrolyzing trichlorosilane in a benzenesulfonic acid hydrate hydrolysis medium and then washing the resultant resin with water and/or aqueous sulfuric acid. Similarly, U.S. Patent No. 5,010,159, describes an alternative method comprising hydrolyzing hydridosilanes in an arylsulfonic acid hydrate hydrolysis medium to form a resin which is separated, washed and contacted with a neutralizing agent. Other hydridosiloxane resins, such as those described in U.S. Patent No. 4,999,397 are produced by hydrolyzing an alkoxy or acyloxy silane in an acidic, alcoholic hydrolysis medium. Any other equivalent hydridosiloxane will also function herein, such as those covered by JP-A [Kokai] Nos. 59-178749, 60-86017 and 63-107122. In a preferred embodiment of the invention, specific molecular weight fractions of the above H-resins may also be used in this process. Such fraction and methods for their preparation are taught in U.S. Patent No. 5,063,267. Alternative approaches, however, are also contemplated. For example, fractions can also be obtained by a non-solvent precipitation process wherein the H-resin is dissolved in a non-polar solvent such as toluene followed by adding a polar solvent such as acetonitrile thereto to precipitate a fraction of the resin. Since this precipitation process progressively precipitates lower molecular weight fractions with higher concentrations of the polar solvent, the process often involves first precipitating and removing the undesired higher molecular weight fractions from the solution followed by precipitating and collecting the desired fraction and leaving the undesired low molecular weight fractions in solution. A preferred fraction comprises material wherein at least 75% of the polymeric species have a weight average molecular weight above 1200 and a more preferred fraction comprises material wherein at least 75% of the polymeric species have a weight average molecular weight between 1200 and 50,000 (relative to polydimethylsiloxane standards). According to the present invention, the H-resin is merely dissolved in a solvent to form a solution. Various facilitating measures such as stirring and/or heating may be used as necessary. Solvents which may be used include any agent or mixture of agents which will dissolve the H-resin to form a homogenous liquid mixture. These solvents include alcohols such as ethyl or isopropyl, aromatic hydrocarbons such as benzene or toluene, alkanes such as n-heptane or dodecane, ketones such as methylisobutylketone, esters, glycol ethers or cyclic dimethylpolysiloxanes. Particularly preferred herein is methylisobutylketone. The above solvents are used in an amount sufficient to dissolve the H-resin to the concentration desired for application. Generally, enough of the above solvent is used to form a 0.1-50 wt. percent solids solution (i.e., 100 parts by weight solvent and 0.1 to 100 parts by weight H-resin). As noted above, H-resin solutions of the prior art may be unstable and undergo molecular weight shifts which may not be desirable (e.g., the material may deposit a thicker coating or the material may partially or completely gel). This instability is dependant on factors such as water, impurities, atmospheric humidity, fractionation and temperature. For example, fractionated material is stable for 3-4 months under refrigeration (e.g., 0°C.) whereas it is only stable for 1.5 months at room temperature. According to the present invention, it has been discovered that the addition of a small amount of acid to these solutions can dramatically affect their stability and shelf-life. Although not wishing to be bound by theory, applicants postulate that both the acidity and the chemical reactions (e.g., oxidation, chemical complexing and the like) of the acids contribute to the stabilizing of H-resin solutions. The acids which can be used herein are any which have the desired stabilizing effect without adversely affecting the resin. These include inorganic acids such as boric, carbonic, hydrochloric, iodic, nitric, nitrous, phosphoric, phosphorous, sulfuric and sulfurous as well as organic acids such as acetic, benzoic, butyric, citric, formic, lactic, maleic, naphthoic, oxalic, phthalic, picric, propionic, succinic, tartaric, toluic, toluene sulfonic and trifluoroacetic. Of these, nitric acid is particularly preferred. The above acids are added to the solution in an amount effective to stabilize the H-resin. This amount can vary over a wide range depending on the acid and the above factors. Generally, however, the acids are added in an amount of between 0.002 and 4 parts per weight and, preferably, between 0.01 and 0.1 parts per weight, based on the total weight of solution. It should be noted that the order of mixing the acid, solvent and H-resin is not critical. For example, in addition to the order described above, the H-resin may be simply dissolved in a solvent-acid mixture. In addition, the solutions of the present invention may also contain other ingredients such as ceramic oxide precursors. Examples of such precursors include compounds of various metals such as aluminum, titanium, zirconium, tantalum, niobium and/or vanadium as well as various non-metallic compounds such as those of boron or phosphorous. These precurors may be dissolved in solution, hydrolyzed and subsequently pyrolyzed, at relatively low temperatures and relatively rapid reaction rates to form ceramic oxide coatings. The above ceramic oxide precursor compounds generally have one or more hydrolyzable groups bonded to the above metal or non-metal, depending on the valence of the metal. The number of hydrolyzable groups to be included in these compounds is not critical as long as the compound is soluble in the solvent. Likewise, selection of the exact hydrolyzable substituent is not critical since the substituents are either hydrolyzed or pyrolyzed out of the system. Typical hydrolyzable groups include alkoxy such as methoxy, propoxy, butoxy and hexoxy; acyloxy such as acetoxy or other organic groups bonded to said metal or non-metal through an oxygen such as acetylacetonate. Specific compounds, therefore, include zirconium tetracetylacetonate, titanium dibutoxy diacetylacetonate, aluminum triacetylacetonate and tetraisobutoxy titanium. When the H-resin solution is to be combined with one of the above ceramic oxide precursors, generally it is used in an amount such that the final ceramic coating contains 0.1 to 30 percent by weight of the modifying ceramic oxide. The solution may also contain a platinum, rhodium or copper catalyst to increase the rate and extent of conversion to silica. Generally, any platinum, rhodium or copper compound or complex which can be solubilized will be functional. For instance, a composition such as platinum acetylacetonate, rhodium catalyst RhCl 3 [S(CH 2 CH 2 CH 2 CH 3 ) 2 ] 3 , obtained from Dow Corning Corporation, Midland, Michigan, or cupric naphthenate are suitable within this invention. These catalysts are generally added in an amount of between 5 to 1000 ppm, preferably 5 to 500 ppm, platinum, rhodium or copper based on the weight of H-resin. The resultant stabilized solutions are useful for forming coatings on various substrates, especially electronic devices. The following non-limiting example is provided so that those skilled in the art will more readily understand the invention. Example Hydrogen silsesquioxane resin was made by the process described in US Patent No. 3,615,272. Generally, the process comprised slowly adding trichlorosilane to an agitated toluene sulfonic acid hydrate hydrolysis medium. After hydrolysis was complete, the hydrolysis medium was settled and the organic layer (containing the H-resin) was separated and removed. The organic layer was washed twice with sulfuric acid and twice with water and then filtered and stripped to about 20 wt% solids in toluene. The H-resin solution was then fractionated by adding acetonitrile to the solution to precipitate out the undesired high molecular weight fractions. The desired molecular weight fraction was then precipitated out of the solution by adding additional acetonitrile. Two samples were prepared in the above manner and their properties are listed in Table 1 at time 0 (Samples A and B). The resultant resins were then treated with the acids and their concentrations listed in Table 1 (with a control). Gel Permeation Chromatography (GPC) was run on the resins (relative to a polydimethylsiloxane standard) at the times listed in the Table (elapsed time) and the results recorded. As is evident from the data, the addition of an acid to the resin solution clearly inhibits the former molecular weight shifts of the art which were indicative of instability.

Claims: A method of stabilising a hydrogen silsesquioxane resin solution comprising 0.1 to 100 parts by weight hydrogen silsesquioxane resin in 100 parts by weight solvent characterised in that 0.002 to 4 parts by weight acid are added to said resin solution. A method according to claim 1 wherein at least 75% of the polymeric species of the resin have a weight average molecular weight above 1200. A method according to claim 2 wherein at least 75% of the polymeric species of the resin have a weight average molecular weight between 1200 and 50,000. A method according any of claims 1 to 3, wherein the solvent is selected from alcohols, aromatic hydrocarbons, alkanes, ketones, esters, glycol ethers and cyclic dimethylpolysiloxanes. A method according to any of claims 1 to 4, wherein the acid is selected from boric, carbonic, hydrochloric, iodic, nitric, nitrous, phosphoric, phosphorous, sulfuric and sulfurous acids. A method according to any of claims 1 to 4, wherein the acid is selected from acetic, benzoic, butyric, citric, formic, lactic, maleic, naphthoic, oxalic, phthalic, picric, propionic, succinic, tartaric, toluic, toluene sulfonic and trifluoroacetic acids. A method according to any of claims 1 to 6, wherein the acid is added in an amount between 0.01 to 0.1 parts by weight. A method according to any of claims 1 to 7, wherein the resin solution further comprises modifying ceramic oxide precursors comprising a compound containing an element selected from titanium, zirconium, aluminum, tantalum, vanadium, niobium, boron and phosphorous wherein the compound contains at least one hydrolyzable substituent selected from alkoxy or acyloxy and the compound is present in an amount such that the coating contains 0.1 to 30 percent by weight modifying ceramic oxide. A method according to any of claims 1 to 7, wherein the resin solution further comprises a platinum, rhodium or copper catalyst in an amount of between 5 to 1000 ppm platinum, rhodium or copper based on the weight of hydrogen silsesquioxane resin.

Inventors: BRODERICK DENNIS WILLIAM, HELWICK JAMES ANTHONY, NAKAMURA TAKASHI, BRODERICK, DENNIS WILLIAM, HELWICK, JAMES ANTHONY, NAKAMURA, TAKASHI
Applicants: DOW CORNING, DOW CORNING TORAY SILICONE, DOW CORNING CORPORATION, DOW CORNING TORAY SILICONE CO., LTD.
Application year: 1994
Main classification: C08G77
IPC codes: C09D183, C08K5, C08G77, C08K3, C08L83